Fix trac #15702, as a followon to fix for #13704.

Summary:
The effect of this change is that -main-is changes the default
export list for the main module, but does not apply the same
change to non-main modules.  This fixes some cases where -main-is
was used to wrap a module that expected that default behavior
(exporting `main`, even when that wasn't the main entry point
name).

Reviewers: mpickering, monoidal, bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #13704, #15702

Differential Revision: https://phabricator.haskell.org/D5322

6 files changed, 44 insertions, 14 deletions

diff --git a/compiler/typecheck/TcRnExports.hs b/compiler/typecheck/TcRnExports.hs
index 1b57608a41..a2f892ba1e 100644
--- a/compiler/typecheck/TcRnExports.hs
+++ b/compiler/typecheck/TcRnExports.hs
@@ -127,9 +127,11 @@ tcRnExports explicit_mod exports
         -- In interactive mode, we behave as if he had
         -- written "module Main where ..."
         ; dflags <- getDynFlags
+        ; let is_main_mod = mainModIs dflags == this_mod
         ; let default_main = case mainFunIs dflags of
-                 Just main_fun -> mkUnqual varName (fsLit main_fun)
-                 Nothing       -> main_RDR_Unqual
+                 Just main_fun
+                     | is_main_mod -> mkUnqual varName (fsLit main_fun)
+                 _                 -> main_RDR_Unqual
         ; let real_exports
                  | explicit_mod = exports
                  | ghcLink dflags == LinkInMemory = Nothing
diff --git a/docs/users_guide/bugs.rst b/docs/users_guide/bugs.rst
index 0290622b62..96cdd25f5a 100644
--- a/docs/users_guide/bugs.rst
+++ b/docs/users_guide/bugs.rst
@@ -171,35 +171,51 @@ same context. For example, this is fine: ::
       g :: Ord a => a -> Bool
       g y = (y <= y) || f True
 
-.. _infelicities-Modules:
+.. _infelicities-default-exports:
 
 Default Module headers with -main-is
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-The Haskell2010 report specifies in <https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-990005.1> that
+The Haskell2010 Report specifies in <https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-990005.1> that
 
     "An abbreviated form of module, consisting only of the module body,
      is permitted. If this is used, the header is assumed to be
      `module Main(main) where`."
 
+GHC's ``-main-is`` option can be used to change the name of the top-level entry
+point from ``main`` to any other variable.  When compiling the main module and
+``-main-is`` has been used to rename the default entry point, GHC will also use
+the alternate name in the default export list.
+
 Consider the following program: ::
 
     -- file: Main.hs
     program :: IO ()
     program = return ()
 
-Under the report, this would fail with ``ghc -main-is Main.program Main.hs``
-with the following errors: ::
+GHC will successfully compile this module with
+``ghc -main-is Main.program Main.hs``, because the default export list
+will include ``program`` rather than ``main``, as the Haskell Report
+typically requires.
+
+This change only applies to the main module.  Other modules will still export
+``main`` from a default export list, regardless of the ``-main-is`` flag.
+This allows use of ``-main-is`` with existing modules that export ``main`` via
+a default export list, even when ``-main-is`` points to a different entry
+point, as in this example (compiled with ``-main-is MainWrapper.program``). ::
 
-    Main.hs:1:1: error:
-        Not in scope: 'main'
-        Perhaps you meant 'min' (imported from Prelude)
+    -- file MainWrapper.hs
+    module MainWrapper where
+    import Main
 
-    Main.hs:1:1: error:
-        The main IO action 'program' is not exported by module 'Main'
+    program :: IO ()
+    program = putStrLn "Redirecting..." >> main
+
+    -- file Main.hs
+    main :: IO ()
+    main = putStrLn "I am main."
 
-GHC's flag '-main-is' allows one to change the entry point name so that
-the above example would succeed.
+.. _infelicities-Modules:
 
 Module system and interface files
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/testsuite/tests/module/T13704.hs b/testsuite/tests/module/T13704a.hs
index 38b1cb73fc..38b1cb73fc 100644
--- a/testsuite/tests/module/T13704.hs
+++ b/testsuite/tests/module/T13704a.hs
diff --git a/testsuite/tests/module/T13704b1.hs b/testsuite/tests/module/T13704b1.hs
new file mode 100644
index 0000000000..28b8800706
--- /dev/null
+++ b/testsuite/tests/module/T13704b1.hs
@@ -0,0 +1,4 @@
+main :: IO ()
+main = return ()
+
+-- wrapped by T13704b2.hs
diff --git a/testsuite/tests/module/T13704b2.hs b/testsuite/tests/module/T13704b2.hs
new file mode 100644
index 0000000000..d76206fe42
--- /dev/null
+++ b/testsuite/tests/module/T13704b2.hs
@@ -0,0 +1,7 @@
+module T13704b2 where
+import Main (main)
+
+program :: IO ()
+program = main
+
+-- meant to be compiled with '-main-is T13704b2.program'
diff --git a/testsuite/tests/module/all.T b/testsuite/tests/module/all.T
index dbba44f0ba..33ce3aeefe 100644
--- a/testsuite/tests/module/all.T
+++ b/testsuite/tests/module/all.T
@@ -284,4 +284,5 @@ test('T11970B', normal, compile_fail, [''])
 test('MultiExport', normal, compile, [''])
 test('T13528', normal, compile, [''])
 test('T13622', normal, compile, [''])
-test('T13704', normal, compile, ['-main-is Main.program'])
+test('T13704a', normal, compile, ['-main-is Main.program'])
+test('T13704b', [], multimod_compile, ['T13704b1.hs T13704b2.hs', '-main-is T13704b2.program -v0'])